1. Field of the Invention
The invention relates to electron discharge devices in which radio frequency energy is generated or amplified by high speed electrons traveling in a beam along the axis of a waveguide and subjected to a periodic magnetic field. The periodic magnetic field causes the electrons to travel in periodic orbits in which electromagnetic energy is radiated. With suitable adjustments of the parameters of the system, energy in the beam is converted into electromagnetic energy of a given frequency. The invention also relates to an improved magnetic structure for deflecting the beam and focusing it in such electron discharge devices.
2. Description of the Prior Art
Electron discharge devices of the type described above have taken a variety of forms, have been given a variety of names, and have been subjected to differing analyses in the literature.
A survey of this class of devices was undertaken by J. Mark Baird in a talk given at the December, 1979 IEEE International Electron Devices Meeting held at Washington, D.C. The resulting paper, which was entitled "Survey of Fast Wave Tube Developments" contained a bibliography of over 40 citations (CH1504-0/79/0000-0156500.75 c 1979 IEEE).
An article appearing in the IEEE (IRE) Transactions on Electron Devices, October 1960, pages 231-241, entitled "The Ubitron, a High Power Traveling Wave Tube Based on a Periodic Beam Interaction in Unloaded Waveguide", by R. M. Phillips, describes a related device. In the device described in the Phillips' article, an electron is subjected to a periodic transverse magnetic field causing the beam to follow a sinuous path. The periodic magnetic field is produced by magnetic means referred to as an "undulator". In the Phillips' article, the device has been analogized to a traveling wave tube, modified in the use of a fast wave rather than a slow wave interaction.
An article dated April, 1971, Journal of Applied Physics, Vol. 42, No. 1, pages 1906-1913, entitled "Stimulated Emission of Bremsstrahlung in a Periodic Magnetic Field" by John M. J. Madey, also treats a related device. The device analyzed employs an array of magnets to produce a circularly polarized magnetic field, which induces a helical shape in an electron beam. Somewhat similar principles have been employed in another arrangement described by Madey in which bifilarly wound helical electromagnets also produce a helical electron beam. Superconducting techniques were used in the later arrangement to achieve the high currents necessary to produce adequate transverse fields. The above Madey implementations have been used to generate energy at infrared and optical wavelengths, using electron beams with energies in the megavolt range. In a Madey U.S. Pat. No. 3,822,410, disclosing a related device, the production of energy in the infrared, optical, ultraviolet and X-ray regions was suggested. The Madey devices, due to their higher frequencies, use optical resonator techniques rather than waveguide techniques to transfer the electromagnetic energy. Since the beams employed are highly energetic, focusing in the interaction structure has been unnecessary.
Devices exemplified by the Madey reference operate at such high electron velocities that defocusing of the beam is not significant. In devices adapted for use at lower voltages, such as the Phillips "Ubitron", defocusing does occur in the interaction structure as a result of the lower velocity of the electrons and space charge effects. If the interaction structure is prolonged, focusing is essential to efficient operation.
It has been proposed to characterize the devices treated above by the term "free electron LASER". The term "LASER" (Light Amplification, Stimulated Emission Radiation) has ordinarily been reserved to devices operating with electromagnetic waves in the visible or near visible (IR) portions of the spectrum.
The device herein treated bears a significant theoretical similarity to a LASER taking into account the theoretical analogy between a microwave resonant structure and an optically resonant structure, but differs in the practical ways of handling the resultant electromagnetic radiation. In the sense herein intended, the term "free electron LASER" is applied to devices in which the practical application is in generation of waves in the millimeter or sub-millemeter region of the electromagnetic spectrum.